A semiconductor device that includes a bidirectional Zener diode is disclosed in Patent Document 1 (Japanese Patent Application Publication No. 2001-257349). The semiconductor device includes an n+ type semiconductor substrate. An n− type epitaxial layer is formed on the semiconductor substrate. A trench is formed in a surface layer portion of the epitaxial layer. A gate oxide film is formed on an inner wall of the trench. A bidirectional Zener diode, including an n+ type region, a p type region, and an n+ type region, is formed in the trench.